There are many publications and patents which disclose the possibility for reconstructing an object in three dimensions on the basis of photographs obtained in two dimensions. This is in particular the case of stereovision. Stereo-radiography, or multi-planar radiography, is based on the same principles, the third dimension is given by the mapping of at least two views taken at different angles of incidence and this mapping is not possible without a calibration stage which, in stereovision, is most often carried out once and for all before the images are taken. In the field of radiography, these different angles of incidence impose either the use of several X-ray sources or a spatial displacement of a single source or a displacement of the subject to be radiographed in front of this single source.
Now, in the field of clinical radiography, the simultaneous use of two X-ray sources is extremely rare. Furthermore, the position of the X-ray source must be adjusted for each subject to be radiographed.
One example is the three-dimensional representation of the human vertebral column. The structural, positional and morphometric information that such representations offer is particularly interesting. It can for example help a surgeon in his pre-operative and post-operative diagnosis or it can be of the type favouring computer-aided surgical operations.
The only information of this type that a surgeon can easily gather comes from stereoradiographic systems that are available only in a very small number of hospitals. The equipment generally consist of a source and a radiographic film and a turntable upon which the patient is disposed in such away as to obtain several views at different angles of incidence. A calibration object is also added to the equipment.
It is in fact necessary to calibrate these views in such a way as to identify the geometrical parameters of the radiographic environment, that is to say to specify the relative positions of the X-ray source with respect to the film.
There are numerous methods of calibration but these generally depend on the description of the relative positions of the images and of the X-ray sources (or of the cameras). In order to determine the radiographic environment, these methods necessitate the management of 18 variables when carrying out calculations. In this case, the calibration object must surround the totality of the measurement volume which makes it particularly voluminous and bulky.
Furthermore, the patient must also be placed inside this measurement volume and can be made uncomfortable and put under stress due to the structure of the calibration object which surrounds him and/or to the length of the measuring and acquisition process.
It has also been proposed to place the patient lying down on a table, as if to scan him. However, unlike in the upright position, this measurement is not carried out under load and the curvatures of the vertical column (physiological or pathological) are thereby modified.
Furthermore, in order to obtain two views at different angles of incidence with only one image generating system, it is possible resort to a translation of the source (vertical or horizontal). But the two angles of incidence obtained are often close, which generates more errors in the 3D reconstruction. Moreover, the photographs thus obtained do not correspond to the radiographs currently used in clinical routine.